Better Handle Low-Fee Paths when Routing

[TheBlueMatt]

Our router currently uses max(htlc_min, fee) for fees when picking a path, which addresses an important edge case where we might reduce a path's amount later during pathfinding and pay up to htlc_min in fees. However, when we're dealing with a path that has a bunch of nodes that have htlc_min of 1 sat and we're sending a small amount (eg a zap of 21 sats), this can start to impact our ability to pick cheap paths.

Its not clear exactly what to do here, but at a minimum we could detect cases where the per-path min means we won't be limited by the htlc_min and can prefer paths by their real fee, rather than just prop fees, when picking paths out of a candidate set during MPP.